Torque ripple reduction for interior permanent magnet machines using overlapping windings with fractional slot per pole pair

Torque ripple is a major concern in interior permanent magnet (IPM) machines, which is usually above 20% in conventional IPM machines, and for some cases, such as spindle motors, this value is too high to be accepted. In this paper, armature windings with fractional-slot per pole pair are employed to reduce the torque ripple. First, it is theoretically explained why this windings are helpful to lower torque ripple without the cost of average torque. Then, the effect of slot pole combinations on torque ripple is investigated. Moreover, the electromagnetic performance, such as back EMF, cogging torque, and torque ripple of an IPM machine with the proposed windings is compared to that of the IPM machine with conventional integral-slot distributed windings (ISDW). It is shown that the cogging torque of IPM machine with fractional-slot per pole pair windings could be as low as 0.2%, and torque ripple is only 3% without any other special measures such as skew or dummy slots.